This invention relates generally to assembling integrated circuit dice into packages.
For a number of reasons, it may be desirable to package two dice together in a single package. For example, the components provided in a separate die may not be capable of being integrated in a single integrated circuit. For example, different process technologies may be used in each die. In addition, basic incompatibilities between components on each die may necessitate separate die fabrication. Once the dice are fabricated independently, it may be desirable to package them in a single package so that a single set of contacts may be used to connect with each of the dice.
The handling of dice, which may be stacked, to form a composite creates a number of difficulties. Predominantly, semiconductor assembly equipment is adapted to produce packages containing a single die. Thus, techniques are needed to assemble the dice.
Conventional dice assembly techniques may involve a pair of passes through a die attach machine. For example, a larger of two dice to be stacked may be attached to a support structure in a die attach machine. Thereafter, the support structure may be run through the die attach machine again to attach the smaller die to a larger die.
The repeated passes through the die attach machine may be disadvantageous for a number of different reasons. For example, the assembly process is complicated by the multiple passes through the die attach machine. In some cases the die attach process may be relatively expensive. In some cases the thermal budget may be impacted by the die attach process.
Die attach machines usually use an organic-based adhesive to attach the support structure to the die or to attach one die to another die. The adhesive may be applied in an uncured or soft phase between the surfaces to be joined. The adhesive is then cured through exposure to elevated temperatures for a specified time period. The carrier material is typically an epoxy resin or polyimide. The carrier provides the adhesion and mechanical strength along the bond line. The carrier may be filled with metal particles in the event electrical and thermal conductivity is required or with non-metallic particles such as alumina, if the ultimate bond should be an insulator.
Three types of polymer adhesives that are often used in die attach processes are epoxies, cyanate esters and polyimides. Epoxies use relatively low cure temperatures (125-175.degree. C.) and have moderate glass transition temperatures (100-155.degree. C.). However, epoxies vary in their ability to absorb and outgas moisture and other contaminants. Cyanate ester adhesives use higher cure temperatures (300.degree. C.) and have relatively higher glass transition temperatures (240.degree. C.). However, unlike epoxy adhesives, cyanate esters have low outgassing characteristics as well as low ionic contents. Finally, polyimide adhesives use high cure temperatures (greater than 400.degree. C.) and like cyanate esters have a high glass transition temperature. A variety of conductive fillers including silver, gold or copper and non-conductive fillers, such as alumina, diamond, glass fabric, silica or ceramic may be used with the adhesive.
Thus, in addition the complexity inherent in the die attach process, in some cases the thermal exposure may be disadvantageous both to shallow junctions in advanced devices and to the wire bonds or other elements whose electrical performance may be affected.
Circuits that are susceptible to outgassing and moisture generally use a metallurgical attachment technique. Solder or metal eutectic alloys may provide good thermal conductivity but because of their electrical conductivity they may not be used if the backside of the die must be insulated from the support structure. Solders and eutectic alloys generally use higher processing temperatures and thereby impose constraints on work flow. For example, if a gold-tin die attach operation is scheduled after other chips in a multi-chip module have been wire bonded, the high temperatures used may degrade the wire bonds or degrade the electrical performance of the actual device.
Thus, for a variety of reasons, it may be desirable in making multi-chip modules, to reduce the number of times that the module or any part thereof must be exposed to the die attach process.